The comics are in some ways very different than the myths. In the myths Odin and Loki are blood brothers.
In the comics, Hela is sometimes stated to be the daughter of Loki and other times to be the daughter of Odin. The former tends to be much more common in modern days. The main contention is for his final test for Mjolnir he had to save Sif from the goddess of death, Hela. This was one of Thor's earliest adventures and sometimes Thor is depicted as a teenager when it happens. So Loki would be too young to be her father. Other stories depict Thor as a bit older so it would be less problematic.

well in the original myth, loki and odin are more like blood brothers. hela is ideed the daughter of loki. in fact loki three children are pretty the end all and be all of powerful monsters in norse mythology

Loki is traditionally stated to be Hela's biological father. However, certain things in the comics seem to contradict this.

Are Loki and Hela biologically related? I never quite understood that.

Loki is the child of frost giants (the big blue guys), but is a runt in comparison to the rest. He was adopted by Odin as a child. Hela is normally portrayed as closer to the size of the rest of the gods, but Yost and Kyle stated they purposely made her huge to show of her giant heritage.
As to why they are human looking, giants look very different depending on who is drawing them.
Loki is much more powerful than what he is shown in this film. He can do anything the Enchantress can do except on a much larger level. This film depicted both Loki and Thor underpowered compared to the Hulk and what they should have been. Hence the many complaints on that aspect of it.
Her mother is a female giant/monster named Angrboa. She has only had once brief appearance in the comics.